Monopropellant comprising an alkyl nitrate and a nitroparaffin additive



United States Patent 3,208,891 MONOPROPELLANT COMPRISING AN ALKYLNITRATE AND A NITROPARAFFIN ADDITIVE Paul C. Eschrich, Wallingford, andGlenn R. Dixon, Hamden, Conn., assignors to Olin Mathieson ChemicalCorporation, a corporation of Virginia No Drawing. Filed Mar. 2, 1964,Ser. No. 348,881 6 Claims. (Cl. 149-91) This application is acontinuation-in-part of application Serial No. 255,694, filed Feb. 1,1963, which was forfeited.

This invention relates to a propellant composition and more particularlyto an alkyl nitrate liquid propellant for energizing devices of theexplosive actuated type.

It is sometimes desirable to make a liquid propellant more sensitive toignition in order to decrease the energy requirements to ignite it. Thisis especially true for the alkyl nitrate propellants in adverseenvironmental conditions. The addition of small amounts of anitroparaffin such as tetranitromethane alternatively referred to as TNMor hexanitroethane alternatively referred to as HNE to alkyl nitratepropellants such as 60 mole percent ethyl nitrate-40 mole percent propylnitrate, or N-propyl nitrate substantially reduces the energy requiredto ignite the propellant by adiabatic compression and by electric sparkdischarge.

One object of the invention is to provide a suitable alkyl nitrate mixcapable of being readily ignited by direct electric spark or byadiabatic compression ignition, in contradistinction to ignition bypriming and detonator explosives set off percussivelyor by an electricbridge wire.

Another object is to provide such an alkyl nitrate mix having not onlythe requisite sensitivity and reliability, but also the safe progressiveburning character needed for discharging a shotgun, or for actuating atool of the explosive actuated type such as a fastener driving tool.

Other objects and advantages will be apparent from this description ofdetails of the invention in alkyl nitrate propellant modified byaddition of a nitroparaflin additive to render the nitrate, which isquite resistant to ignition by electric spark or adiabatic compression,ignitable by such a spark or compression in any suitable ignitionchamber, particularly when the propellant is loaded in a preferreddensity range with respect to the volume of the chamber.

Any nitroparaflin may be used in accordance with the present invention.Preferably, the nitro-lower-parafiins are employed, i.e. those saturatedaliphatic hydrocarbons 0f the methane series C H containing less than 7carbon atoms and optimally TNM or I-INE. The amount of nitroparaffinwhich may be used is at least 0.5% by weight. Preferably less than 15%by weight is employed and still more preferably a range of 1% to 10% byweight of the nitroparafiin is used.

One of the various lower alkyl nitrates contemplated in accordance withthis invention, such as ethyl nitrate, isopropyl nitrate, normal propylnitrate, amyl nitrate, hexyl nitrate, and the like, a mixture of ethylnitrate and n-propyl nitrate in about a 60:40 mole ratio, has been foundvery promising for the purpose, provided there is incorporated a smallbut suflicient amount of tetranitromethane, a liquid nitroparaflincomponent providing the advantage ofimproved ignitability whileimproving the oxygen balance of the whole mixture because it is acompound with up to 48% available oxygen.

It is preferred in accordance with the present invention to utilize amixture of lower alkyl nitrates. Naturally all proportions arecontemplated. The preferred embodiment of the present invention asstated above is the mixed ethyl-n-propyl nitrates. The preferred ratioof ethyl to n-propyl is from 3:1 to 1:1.

Specifically, a 60:40 mixture of the mixed ethyl-npropyl nitrates havinga boiling range of from about 190 F. to 231 F., and a freezing pointbelow 148" F., at standard conditions of temperature and pressure, aclosed cup flash point of about 50 F., and a density of about 1.085grams/cc. is contemplated for modification according to this invention.A mixture containing somewhat more ethyl nitrate, e.g. as high as butpreferably less than mole percent, and requiring a bit lesstetranitromethane or other nitroparaflin additive, is contemplated aswell as one containing somewhat less ethyl nitrate such as one having atleast 50 mole percent in n-propyl nitrate and requiring somewhat more ofthe additive. The percent by weight figures for tetranitromethane aregiven herein for the 60:40 mixed ethyl propyl nitrate, hereinafterreferred to as 60:40 EPN.

With the additive, the 60:40 EPN has about the same density as thatnoted above. This is true within the limits of mixed liquid nitrates,contemplated such as the 70:30 EPN and 50:50 EPN.

Tests carried out with a fuel comprising a single alkyl nitrate such asn-propyl nitrate or comprising a mixture of alkyl nitrates such as 60:40ethyl-n-propyl nitrate clearly indicate that the addition of a smallamount of a nitroparafiin such as TNM or HNE appreciably improves theignition characteristics of the alkyl nitrate or mixed alkyl nitrateseither by electric spark ignition or by adiabatic compression ignition.Generally, the greater the percentage by weight of nitroparafiin addedto the alkyl nitrate fuel, the greater will be the effect on improvedignition.

Based on the test data obtained, it would be expected that the additionof a nitroparafiin would improve the ignition sensitivity of most loweralkyl nitrates or mixtures thereof in all proportions, and at alltemperature ranges, and in all environmental conditions.

It is contemplated that the monopropellant mixture of this invention isto be used in an ignition system in the presence of a gaseous phase,i.e. at loading densities less than 1.085 grams/co, and that the systemmay in one embodiment be in the form of a blank cartridge, shot shellcartridge, or the like, and in another embodiment the monopropellant maybe put into a chamber for ignition independently of any cartridge.

Other additives compatible with the fuel mix such as nitroglycerin andethylene oxide are also contemplated which may be used as well as thenitroparafiin.

This invention will be more clearly understood in connection with thefollowing examples which are given as illustrations of the invention andnot as limitations.

EXAMPLE I Advantageous ignition characteristics were found when variousamounts of tetranitromethane; herein after referred to as TNM, wereadded to 6:40 EPN at various loading densities in a closed sparkignition chamber utilizing a spark gap of about 0.095 of an inch, atypical dimension in the range of about 0.1 of an inch understood to bevariable downwardly to a slight extent to further reduce the voltage andenergy requirements to a limited extent for safety and ease of ignition.Data obtained under the conditions described above is shown in Table Ibelow.

Table I Mixture Wt. Percent Loading Energy wt. percent (30/40 EPNdesnity (joules) TN M (g./cc.)

The data obtained from Table I can be summarized as follows:

At loading densities in the range from about 0.3 gram/ cc. to about 0.4gram/cc., mixtures having a TNM content ranging from about 2 per cent toabout 6 per cent by weight in 60:40 EPN have distinct advantage. Forexample, by using a mixture of 6 percent TNM and 94 percent of the 60:40EPN, with the mixture at around a 0.4 gram/ cc. loading density in thechamber, there was experienced the greatest reduction in voltagerequired and energy required.

From about 4 to about 6 percent, TNM is remarkably effective as anignition aid in 60:40 mixtures loaded to densities over a broader rangeof from about 0.3 or a bit less to about 0.5 gram or a bit more percubic centimeter of chamber. Desirable characteristics are also obtainedin this range from 0.3 gram/cc. to 0.5 gram/cc. loading density, withthe 60:40 EPN containing from 3 to 4 percent TNM. Optimum performanceoccurred at a loading density of 0.40 gram/ cc. with 60:40 EPN having 4percent TNM with respect to economical use of TNM, and ease of ignitionwithout too much sensitivity for safe handling.

At a loading density of about 0.3 gram per cubic centimeter, a 4 percentaddition was found to reduce the energy required to less than 5 joules;at the same density a 6 percent addition lowered the energy required toabout 3 joules. At a loading density of about 0.5 gram/cc. the energyrequired was a little over 4 joules and a little less than 3 joules withadditions of 4 percent and 6 percent of TNM, respectively. Withadditions of from about 3 percent to about 4 percent TNM, the electricalenergy required was between 3 and 4 joules at a monopropellant loadingdensity of about 0.4 gram per cubic centimeter.

Additions of a few percent TNM such as about 2 percent TNM in themonopropellant, when loaded to a density of from about 0.25 to about 0.4permitted spark ignition at an energy represented by less than -12joules; with a 60:40 mix having 2 percent additive, ignition wasachieved at an energy as low as about 5 joules. In any event, withmonopropellant having from about 4 percent to about 6 percent TNM andloaded to suitable densities, ignition is readily achieved electricallyat energy levels as low as between 2 and 4 joules.

Additions of from about 1 percent to about 8 percent TNM is Plated, Fora 50:50 EPN system as high as 8 percent is contemplated. Less TNM isrequired for a 70:30 EPN system, for comparable results. Additions offrom about 3 percent to about 5 percent are preferred in the above-notedoperative ranges of ignition chamber loading density. Addition of toomuch tetranitromethane, produces a detonation rather than progressiveburning upon ignition.

Addition of a nitroparaffin such as TNM, in the disclosed limits, makespossible a reduction of electrical energy required by a factor of atleast ten; it provides the basis for direct ignition in a simple andeconomic way, free from undesired side effects of intervening secondaryexplosives of percussion and thermal sensitive primers.

EXAMPLE II Experiments were performed with a standard compressionignition test apparatus to show the improved sensitivity of alkylnitrate propellants with nitroparaffin additives to compressionignition.

The apparatus consist of a piston, closed chamber, O-ring, steeldiaphragm and falling weight. The O-ring was placed in the closedchamber with the test sample mixture, and the steel diaphragm was placedon the O- ring. On top of this assembly the piston was placed, beingheld at a fixed distance, about 0.050 inch, above the fuel mixture.

The falling weight impacted the piston, driving it down into the chamberagainst the diaphragm and against the O-ring; thereby, effectingadibiatic compression of the air above the fuel mixture which ignitedthe fuel. Upon ignition, the combustion pressure of the fuel burst thediaphragm, and the gases were vented through a small hole in the piston.

In Table II, sensitivity E (kg-cm.) indicates a 50% probability ofignition of the mixture occurring when a given weight is dropped a givendistance. The lower the E (kg-cm.) number the more sensitive to ignitionis the mixture. The data shown in this table is grouped in threesections for the purpose of comparison.

The data obtained in these experiments are summarized in Table II below.

Table II COMPRESSION IGNITION TEST DATA Mixture Tomp., Drop SensitivityF. \vt., kg. E50 (kg.cn1.)

1. (a) 60/40 EPN 70 l 2. 5 ([7) 60/40 EPN, 96%; TNM, 4% 70 l 2.1((1)60/40 EPN, 94%; TNM, 6%- 70 1 1.7 2. (a) n Propyl nitrate V 1 15.9

(b) n-Propyl nitrate, 95%; TNM,

5% 70 V 6. 7 (c) n'Propyl nitrate, 90%; TNM,

10 o 70 2 4. 4 3. (a) 60/40 EPN 0 5. 2 (1)) 60/40 EPN, 90%; TNM, l0% 03. 8 (0) 60/40 EPN, 95%: hexanitroethane, 5% 0 4. 7

1 (1 kg. 11.5). 2 (1 kg. 3.2).

b, and c, it is shown that the addition of small amounts oftetranitromethane (TNM) to a mixture of 60/40 EPN lowers the Esensitivity point. In 2a, b, and c, it is shown that the addition of TNMto a single alkyl nitrate, n-propyl nitrate, reduces its E sensitivitynumber substantially. In 311, b, c, it is shown that at a sub-normaltemperature (0 F.) the E sensitivity of 60/40 EPN is greater than at 70F. and this E sensitivity is decreased by the addition of thenitroparaffins TNM and HNE.

In 1a,

EXAMPLE III Further experiments were carried out to show the improvedsensitivity of alkyl nitrates with nitroparaffin additives in a newcompression ignition apparatus designed to test ignition sensitivity offuel mixtures in a combustion gas atmosphere. The sensitivity toignition is indicated as percent ignition failures at different levelsof energy produced by a falling weight.

The apparatus consist essentially of a closed chamber, a moving pistonwhich always remains in the chamber, and a falling weight. The fuelmixture was injected in the chamber and the piston was set at a fixeddistance above the fuel (.5 inch). The falling weight drove the pistondownward, compressing the atmosphere above the liquid and, thus,igniting the fuel. Upon ignition, the piston was driven upward exposingexhaust ports leading to closed chamber, thereby, trapping the exhaustgases. The next test was then conducted in an atmosphere of thecombustion gases.

Table III shows the data obtained from this test in evaluating theperformance of a TNM 60/40 EPN fuel mixture, a HNE 60/40 EPN fuelmixture and a 60/40 EPN fuel mixture without an additive. The test datais recorded as percent of ignition failures which occurred when a givenweight was dropped from a given height to ignite the propellant mixture.

Table III COMPRESSION IGNITION TEST APPARATUS NO. 2

90% 60/40 EPN; Percent ignition Inch- TNM, failures: 90% 60/40 EPN,pounds percent 60/40 EPN; 10% percent hexanitroethane The difficulty ofmaintaining exactly the same atmosphere from test to test may accountfor the unexpected variation in the experiments using HNE where a smallincrease in percent failures was actually recorded when going from 8 to9 inch pounds. However, maintaining similar procedures in each test,yielded data which clearly shows that less energy is required to ignitefuel mixtures efiiciently with mixtures containing the nitroparaffinadditives of TNM and HNE.

What is claimed is:

1. A monopropellant liquid mixture consisting essentially of a loweralkyl nitrate and a nitroparafiin additive containing 1 to 7 carbonatoms in an amount of 0.5% to 15% by weight, said liquid being readilyignitable by electric spark or by adiabatic compression.

2. A mixture according to claim 1 in which said additive istetranitromethane or hexanitroethane.

3. A monopropellant liquid mixture consisting essentially of ethylnitrate and normal propyl nitrate to which is added a nitroparaflincontaining 1 to 7 carbon atoms in an amount of 0.5% to 15% by weight,sufficient to render said mixture more sensitive to ignition by electricspark or adiabatic compression.

4. The mixture of claim 3 containing to 10% by weight oftetranitromethane or hexanitroethane.

5. The mixture of claim 3 containing more than mole percent but lessthan 70 mole percent ethyl nitrate and containing less than 50 molepercent but more than 30 mole percent normal propyl nitrate.

6. The mixture of claim 3 containing not less than 1% but not more than10% tetranitromethane in a :40 mixture of ethyl nitrate and normalpropyl nitrate.

References Cited by the Examiner UNITED STATES PATENTS 2,338,120 1/44Lawrence 14990 X 2,551,554 5/51 Wood 14990 2,993,768 7/61 Holzl 149-883,116,187 12/63 Scanlon et al. 144-90 X CARL D. QUARFORTH, PrimaryExaminer. BENJAMIN R. PADGETT, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Noe3,208,891 September 28, 1965 Paul c. Eschrich et al It is herebycertified that error appears in the above numbered patent requiringcorrection and that the said Letters Patent should read as correctedbelow.

Column 6, line 18, after "containing" insert M 1% Signed and sealed this5th day of July 1966c (SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner ofPatents

1. A MONOPROPELLANT LIQUID MIXTURE CONSISTING ESSENTIALLY OF A LOWER ALKYL NITRATE AND A NITROPARAFFIN ADDITIVE CONTAINING 1 TO 7 CARBON ATOMS IN AN AMOUNT OF 0.5% TO 15% BY WEIGHT, SAID LIQUID BEING READILY IGNITABLE BY ELECTRIC SPARK OR BY ADIABATIC COMPRESSION. 